Analysis of the performance and service life of structural adhesives

1. Structural adhesives are the controlling materials that affect the life of curtain walls
1.1 The role of the performance of silicone structural adhesives in building durability
The performance and life of silicone structural adhesives have been a topic of long-term concern in the industry, and manufacturers, construction units, direct users and designers are all very concerned. The small adhesive strips are like sewing threads, which elastically connect the building fasteners and also have to withstand the corresponding forces. Like sewing threads, the amount and cost of this material are not large, but the role it plays is very critical. Since most of the main materials of the building are inorganic, the durability of these main materials is generally guaranteed. However, silicone structural adhesives are organic materials, and their durability is not as good as inorganic materials, so silicone structural adhesives should be the controlling and restrictive materials that affect the service life and structural safety of curtain wall buildings.
1.2 Comparative analysis of the advantages and disadvantages of silicone adhesives
Silicone adhesives contain both organic groups and inorganic Si-O structures. This structure makes it have the properties of both inorganic and organic materials, especially in terms of excellent UV resistance, heat resistance and aging resistance. Therefore, it is the preferred material for building curtain walls that can withstand historical tests. It has been used for more than 40 years abroad and 30 years in China.
However, silicone structural adhesives are not as good as C-C polymer compounds in terms of acid and alkali resistance, hydrolysis resistance and chemical inertness. Therefore, under the action of substances such as acids and alkalis, metal catalysts, and water vapor (especially products such as silicone have a high water vapor permeability), silicone structural sealants may undergo chemical reactions to cause aging and performance degradation.
1.3 Analysis of structural adhesive bonding performance
The bonding performance evaluation of structural adhesives involves two important indicators-bonding damage area and bonding strength. The bonding force and bonding durability of structural adhesives must be sufficiently guaranteed. There are some typical manifestations of non-adhesion. If the adhesive strip is peeled off with a light pull, and the peeling surface appears very smooth, without much residual adhesive, in most cases, the pulling strength is sufficient, but the adhesive damage area will exceed the standard. How to solve the bonding problem, there is no universal glue. The current status of the industry is as follows: standard materials such as glass have very good bonding performance, and standard anodized aluminum profiles (bonding is also guaranteed. However, for new materials with various surface treatments, the bonding conditions cannot be guaranteed to be completely stable. Therefore, the current general method is to apply some primer on the bonding surface in a targeted manner. Primer is a solution that has been proven to be very useful for a long time, and another benefit of primer is to improve the bonding condition at the interface and avoid adding too many harmful substances to the entire colloid. In the selection of new materials, both appearance and safety should be taken into account, especially in terms of safety. The construction unit should fully understand and cooperate, the surface should be cleaned, and the construction should be maintained.
1.4 Overview of structural adhesive aging performance
A key word is involved here: chemical aging. Physical aging is one aspect, and chemical aging is more core. The manifestation of aging is mainly the obvious deterioration of properties such as adhesion, tensile strength, elongation at break, and elastic recovery rate. In severe cases, the colloid will become very hard and lose elasticity. Aging also has initial and long-term manifestations. In the initial aging and degradation process, the core cross-linking points of the polymer are first degraded, so the cross-linking density of the glue decreases, the strength decreases, but the basic chain of the polymer is still there, so although its elongation increases, the rebound rate decreases, and aging has begun. But if it is deeply aged, the polymer has slowly decomposed, or even volatilized, and more fillers are left, so the elasticity is very poor and it will powder and harden. my country's tracking and detection of existing curtain walls has been put on the industry's agenda.
2. Scientific research on quality standards of structural adhesives
2.1 Quality standards of silicone structural adhesives in my country
Regarding silicone structural adhesives, we currently have mandatory national standards in China, and the European standards have been discussed a lot in recent years. China's national standards have been implemented for nearly 15 years and have played an irreplaceable role in the development of the industry. But now it seems that there is still a gap between the national standards and the European standards. The gap is mainly reflected in the fact that my country's national standards are mainly used to measure the performance of fresh adhesives under standard conditions, while the amount and degree of observation after the adhesive has aged are not enough. In comparison, the European standard does a better job in this regard. After the aging test, it is seen how much the relative performance of the adhesive can be maintained, and the relative performance retention rate is required to be no less than 75%. This involves the relationship between the short-term and long-term, static and dynamic, absolute and relative values of the adhesive, and emphasizes the maintenance of balanced stability and long-term dynamic performance.

2.2 Comparative study of my country's current standards and European standards

Some test items of the European standard require more than The national standard is high. For example, water needs to be exposed to ultraviolet light for more than 1,000 hours, and the exposure time and intensity are 2-3 times that of the national standard. It can be seen that the European standard pays great attention to aging and lifespan, and pays attention to whether the long-term performance can be maintained during use. The national standard is like the college entrance examination in my country. It is very strict in the entrance examination, but the future development of the child is uncertain. The European standard looks at the longer term.
However, the European standard is not simply stricter than the national standard, but more scientific. For example, the national standard has another standard that is stricter than the European standard, that is, the damage area of the bond cannot be greater than 5%, and the European standard is 10%. I think 5% is not very scientific, because this is a regulation of absolute strength, without considering the bonding conditions and environment. Polymer materials (actually a process of improvement, such strict requirements in the early stage may cause adverse effects.
3. Explanation of the current status of the life of structural adhesives

3.1 The current status of the life of structural adhesives

The study of the life of structural adhesives is a key concept related to building safety. The life of curtain walls is calculated as 100 years. The number of repairs and costs of structural adhesives are very different if the life of the structural adhesive is 15 years or 25 years. According to the current industry propaganda, if the European standard is passed, the life can reach 25 years, and the national standard is 10 to 15 years. According to our understanding of the factors affecting the life, to solve this problem, the formula is the key, and the chemical laws of structural adhesives must be respected. Curtain wall companies should not be too accommodating to short-term construction requirements. Domestic and foreign structural adhesives face the same problem. The proportion of domestic adhesives used in high-rise curtain walls is not very high. There is a gap between China and foreign countries. The degree of research history and details is different, but the aging problems they face are the same. Curtain wall companies hope that the structural adhesives will cure as quickly as possible. In order to cater to customers, the adhesive factory can only add more catalysts to the colloid. However, everyone should know that the catalyst that accelerates the rapid curing of the colloid can also accelerate the rapid decomposition of the colloid. It is a double-edged sword. my country is currently also curtain China is a country with a large number of curtain walls, so safety and stability are also a major issue.
I hope that the entire industry can develop healthily and rationally. In the absence of conclusive experiments and practical tests, we should be cautious in promoting the life of structural adhesives. At present, the statement of 50 years is not appropriate. 25 years is combined with the European standard, but in fact, the European standard has not been officially implemented in my country. Therefore, although the curtain wall has a history of 50 years, it does not mean that its structural performance is still as good as before. I hope that the industry can understand the reality of the national standard and maintain confidence in structural adhesive products.
3.2 Methods to extend the life of structural adhesives
For my country's structural adhesives to reach the world level and allow everyone to use them with confidence, it is inseparable from the efforts of adhesive factories, the scientific specifications of national standards, the guidance and management of industry associations, and the understanding, support and cooperation of upstream and downstream industries. From the perspective of manufacturers alone, we must make in-depth improvements in the two aspects of formulation and testing. From the past, the structural adhesive industry has been under great pressure. From the future, my country's curtain wall industry is still in the development stage. The performance of structural adhesives should not be a sprint but a marathon.